Chair back



FIG. 1 is a front perspective view of a chair back my new design;

FIG. 2 is a rear perspective view thereof;

FIG. 3 is a front elevational view thereof;

FIG. 4 is a rear elevational view thereof;

FIG. 5 is a left side elevational view thereof, the right side being a mirror image; and,

FIG. 6 is a top plan view thereof. 

CLAIM The ornamental design for a chair back, as shown and described. 